4-Methylumbelliferyl phosphate dilithium salt is a fluorescent substrate for the enzyme phosphatase. It is used as a marker to show that the enzyme has been activated and dephosphorylated. 4-Methylumbelliferyl phosphate dilithium salt is a monoclonal antibody that can be used to detect surface antigens of mammalian cells. It also has been shown to bind to endogenous phosphatase in mammalian cells, which may be due to its high affinity for this protein.